Photo of David A. Williams


David A. Williams

Adjunct Professor of Physics
Ph.D. Harvard University, 1987

Office: Room 319, Natural Science 2
Phone: (831) 459-3032
FAX: (831) 459-5777
e-mail: daw@scipp.ucsc.edu

Research Interests

I am a member of the VERITAS, AGIS, Milagro and STACEE collaborations, pursuing the study of high-energy gamma-rays from astrophysical objects. These experiments adapt experimental techniques developed for particle physics to explore outstanding problems in high-energy astrophysics and gamma-ray astronomy. I am particularly interested in understanding the nature of gamma-ray bursts and the high-energy emission mechanism of active galactic nuclei. I participate in both the development of the instrumentation for the experiments, especially the electronics and photodetectors, and in the analysis and interpretation of the data.

Milagro and STACEE have completed data taking and some of the final data analysis is still under way. VERITAS is in its prime data-taking phase and is the project on which I spend the majority of my effort. We have an active program of source observations and are also planning an instrument upgrade to improve the sensitivity. AGIS is a proposed concept for a future gamma-ray instrument with about ten times better sensitivity than VERITAS. Work is underway towards a more detailed design for AGIS and construction of a prototype telescope.

I am also an Affiliated Scientist with the Fermi Large Area Telescope (LAT), with a particular interest in studies combining high-energy gamma-ray data from the LAT with very high-energy gamma-ray data from VERITAS.

Collaboration Involvement: VERITAS AGIS Fermi-LAT Milagro STACEE

Selected Recent Publications

Radio Imaging of the Very-High-Energy γ-Ray Emission Region in the Central Engine of a Radio Galaxy, V. A. Acciari et al., Science, in press (2009).

Milagro Observations of Multi-TeV Emission from Galactic Sources in the Fermi Bright Source List, A. A. Abdo et al., Astrophys. J. Lett. 700, L127 (2009).

VERITAS Observations of a Very High Energy Gamma-ray Flare from the Blazar 3C 66A, V. A. Acciari et al., Astrophys. J. Lett. 693, L104 (2009).

Discovery of Localized Regions of Excess 10-TeV Cosmic Rays, A. A. Abdo et al., Phys. Rev. Lett. 101, 221101 (2008).

VERITAS Discovery of >200 GeV Gamma-Ray Emission from the IBL W Comae, V. A. Acciari et al., Astrophys. J. Lett. 684, L73 (2008).

TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro, A. A. Abdo et al., Astrophys. J. Lett. 664, L91 (2007).

Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy, A. A. Abdo et al., Astrophys. J. Lett. 658, L33 (2007).

Evidence for TeV Gamma-ray Emission from a Region of the Galactic Plane, R. Atkins et al., Phys. Rev. Lett. 95, 251103 (2005).

Complete list of VERITAS publications
Complete list of Milagro publications
Complete list of STACEE publications

More about Research

Particles more energetic than any man-made accelerator can produce are continuously bombarding the earth. We do not yet understand where these high-energy cosmic rays come from or how they are accelerated to such tremendous energies in astrophysical systems. The vast majority are charged particles which are deflected in the galactic magnetic fields, such that their arrival direction tells little about whence they came. Gamma rays, while much rarer, arrive at earth unperturbed, still appearing to come from their true source. By finding and studying sources of high-energy gamma rays, we can identify objects where particle acceleration is taking place and learn about the mechanisms involved.

Low energy photons from the 2.7 K cosmic microwave background or infrared starlight can collide with high-energy gamma rays to produce an electron-positron pair, effectively absorbing some of the gamma rays. For extragalactic sources, the amount of this absorption can probe the amount of intergalactic IR radiation. UCSC theorist Joel Primack and collaborators have been leaders in predicting how measurements of this absorption could distinguish between different cosmological models.

Satellites detect short bursts of low-energy gamma rays about once a day lasting from a fraction of a second to a few tens of seconds. Each gamma-ray burst is from a different location, and no one has yet been able to identify the burst sources with any particular known objects, nor has any burst has yet been detected above 30 GeV. Detection of bursts at TeV (10^12 eV) energies, or a definitive lack of emission, would help constrain models of burst production.

Milagro is a unique air-shower detector in the mountains west of Los Alamos, NM. It uses a 6 million gallon water reservoir equipped with 723 photomultipier tubes under a light-tight cover to detect showers of charged particles produced when gamma rays interact high in the atmosphere. Those relativistic particles from the shower which reach the ground produce Cherenkov light in the water which is detected by the phototubes. Milagro is sensitive to gamma-ray showers above about 100 GeV (10^11 eV), and views a large fraction of the overhead sky at any given moment. It is an ideal instrument to look for phenomena such as gamma-ray bursts in which the direction from which the next burst will come cannot be predicted.

STACEE uses the large array of mirrors at a solar power facility to detect atmospheric Cherenkov light produced by gamma-ray showers in the atmosphere. This technique is able to push the threshold for ground-based gamma-ray detection down below 100 GeV, while maintaining very good sensitivity.

VERITAS is a powerful new system of four 12-m imaging atmospheric Cherenkov Telescopes completed in winter 2007. It has substantially improved sensitivity compared to prior instruments such as STACEE and has already detected a number of new gamma-ray sources.

Other Interests

When not thinking about physics, I enjoy playing violin in the Peninsula Symphony and hiking, e.g., with the local Sierra Club chapter.



Page revised July 2009